Process of preparing improved cast iron articles



4May 21, 1968v A.,D. ACKERMAN ETAL v 3,384,515

PROCESS OF PREPARING MPROVED CAST IRON; ARTICLES Fned'June 21, 1965ATTOY v EY United States Patent O 3,384,515 PROCESS F PREPARING IMPROVEDCAST IRGN ARTICLES Allen D. Ackerman and RoyR. Alliertzart, Saginaw,

Mich., and Arthur I. Siewert, Defiance, Ohio, assignors to GeneralMotors Corporation, Detroit, Mich., a

corporation of Delaware Filed June 21, 1965, Ser. No. 465,388 Claims.(Cl. 14S- 2) ABSTRACT 0F THE DISCLOSURE This invention relates to castiron articles and more particularly to a process whereby excellentdurability and good machinability are achieved simultaneously in acarbidic cast iron article requiring a wear resistant surface.

A specific example of an article to which this invention might beapplied is cast gears. Another example is a camshaft. The surface of thecamshaft lobes must be extremely hard and resistant to wear. The rest ofthe casting must be durable and machinable. Durability as used hereinmeans resistance to wear, scufng, spalling, pitting, fatigue and generaldeteriorization of the surface during service.

Previous to this invention improvement in the durability of a camshafthas been obtained only with a concurrent loss in machinability. This wasbecause durability in a camshaft alloy was obtained by alloying graycast iron with elements which promote the formation of complex ironcarbides. These complex iron carbides in a matrix of fine lamellarpearlite significantly increased the durability of the camshaft but atthe same time made the article very difficult to machine. It wasbelieved that the only way that the article could be made moremachinable was to reduce the content of the complex iron carbides andthus reduce durability simultaneously. The upper limit of how durable acamshaft could be made in the prior art was determined by whether themore durable camshaft could still be drilled, tapped, hobbed, milled,

turned or ground at an economic rate Without excessive tool wear orbreakage.

Thus, it is an object of this invention to provide a durable andmachinable carbidic ii'on casting.

Another object of this invention is to provide a method of producing acarbidic iron alloy article to obtain simultaneously properties ofdurability and machinability.

A more specific object of this invention is to provide a method ofproducing a carbidic iron alloy camshaft to obtain simultaneouslyexcellent durability and good machinability characteristics.

These and other objects are accomplished by casting the articleaccording to a standard technique known in the prior art, cooling,removing the mold, cleaning the casting, annealing at a temperature justbelow the austenite region until incipient spheroidization of thepearlte and until the matrix hardness is less than Rockwell C, cooling,machining and surface hardening by heating 3,384,515 Patented May 21,1968 ICC into the austenite region and quenching. The relative order ofthe machining and surface hardening steps is not critical but willdepend upon the nature of the individual article.

In the figures:

FIGURE l is a photomicrograph depicting the microstructure of a typicalcarbidic cast iron alloy before the heat treatment of this invention.

FIGURE 2 is a photomicrograph depicting the microstructure of a typicalcarbidic iron alloy after the heat treatment of this invention.

The magnication in both the photomicrographs is 750x. In each case thespecimens were etched with a 2% Nital solution.

FIGURE 3 is a camshaft.

Since the camshaft must be highly durable and must have a hard surfaceresistant to wear, gray iron alone is not a satisfactory material forthe casting. Normally one or more carbide forming elements are added tothe gray iron. These elements comprise chromium, vanadium, molybdenum,manganese, columbium, tungsten and zirconium. They are added in smallquantities, each usually less than 2%, and promote formation of hardcomplex iron carbides.

An alloy of the above composition is cast into a cam shaft according toa standard technique known in the prior art. Typically a shell mold maybe used and two or `more camshafts cast in each mold. After cooling themold is removed and the castings are cleaned by sand blasting or othersuitable means.

A camshaft casting containing one or more of the above mentionedalloying elements after cooling to about -a 200u F. will have amicrostructure as depicted in FIG- URE l. This microstructure consistsof flake graphite, pearlite, primary cementite and other metal carbides.In FIGURE l the large white areas indicate the complex iron carbides.The long narrow dark sections depict the ilake graphite. The pearlite atthis stage is in a fine lamellar state and appears as the gray massthroughout the photomicrograph in FIGURE l. While the presence ofpearlite, primary cementite and other carbides make a more durablecasting, the combination of fine lamellar pearlite, primary cementiteand other carbides makes subsequent machining difficult and expensive.However, surprisingly, it is unnecessary to reduce the content of thesecomplex iron carbides and thus reduce durability in order to improvemachinability. The durability can be maintained and the machinabilityimproved by annealing the casting at temperatures below the lowertemperature limit of the austenite region for camshaft alloys for timessufficient to produce incipient spheroidization of pearlite and lowermatrix hardness below 30 Rockwell C.

The actual matrix hardness after annealing will depend upon thecomposition of the alloys. It has been found that most of the hardnessvalues preferably fall in the range of 20-30 Rockwell C. However it isimportant only that the Imatrix hardness should be below 30 Rockwell C.

Temperatures should be as high as possible to allow rapid achievement ofthe incipient spheroidization of pearlite but not so high as to enterthe austenite region and thus develop a hard martensite microstructureupon cooling. The pearlitic structure will tend to spheroidize attemperatures within 20D-300 F. of the lower critical temperatures.However, when the annealing is conducted at the lower end of this rangethe process may be prohibitively slow. A preferred temperature is onewhich is suiiiciently close to the austenite region to permit a rapidbreaking up of the fine hard lamellar pearlite but not so rapid as toprevent good control of the process.

The rate of cooling of the casting from the annealing temperature isdependent upon the size and shape of the article. Stresses caused byuneven cooling are undesirable. It may be necessary to cool largeobjects in the furnace. Objects of smaller cross section, such ascamshafts, may preferably be air cooled.

The microstructure of the casting after the annealing treatment is shownin FIGURE 2. The flake graphite and the carbides are unchanged, but thetine hard lamellar pearlite structure has been broken up renderingmachining significantly casier. It can be observed in FIGURE 2 that thealternate layers of cementite and ferrite which compose the pearlite,are now more discernable and that the cementite layer has started tocontract.

After annealing and cooling the cast iron article may be machined and/orsurface hardened depending upon the nature of the article and theeconomy of a particular production operation. The order in Which thesesteps are performed may depend upon the nature of the article involved.By machining is meant drilling, tapping, hobbing, milling, turning orgrinding. Any such machining is preferably done at an economic ratewithout excessive surface tool wear or breakage.

Surface hardening is typically accomplished by fiame or inductionheating to a temperatur-e within the austenite region of the particularalloy and subsequently rapidly cooling to develop a hard martensitemicrostructure in the surface.

In the case of camshaft 1.0 as shown in FIGURE 3, this sequence ofpost-annealing operations is as follows:

Center holes 14 are located and drilled at either end of the camshaft tofacilitate the lobe hardening heat treatment. The surfaces of the lobes12 are then heated by fiame or by induction to a temperature in theaustenite region of the particular alloy and subsequently cooled rapidlyto develop a hard martensite microstructure in the surface. Otherunhardened surfaces are then machined such as the journals 16 and thegear 18. The lobe surfaces 12 are then finished by a grinding operation.

Specific cast iron compositions of this invention are secondary tomicrostructure and surface hardness obtainable by the post-annealingsurface heat treatment. In a camshaft, the content of hard metalliccarbides and the matrix hardness in the lodes are the factors to becontrolled for good durability.

Metallic carbide content can be kept high by controlling the carbideforming element content (chromium, vanadium, molybdenum, manganese,columbium, tungsten and zirconium) and the chilling tendency of themold. The proper economic balance between these factors that stillproduce a primary complex iron carbide content in excess of 10% byweight is the oneL to be used.

Thus, the subject invention includes the application of the aboveannealing heat treatment to a highly carbidic cast iron alloy articlewhen it is desired to obtain a tough and durable article which can beeconomically machined and if necessary subjected to a surface hardeningheat treatment.

An example of the proper combination of the cornposition factors and anillustration of a specific embodiment of the invention in thepreparation of a camshaft is as follows. Reference may be made to thecamshaft in FIGURE 3.

A cast iron alloy comprised of S-3.50% carbon, 2.20-2.45% silicon,0.60-0.80% manganese, 1.30-1.50% chromium, OAC-0.60% molybdenum,G25-0.40% nickel, less than 0.15% sulphur, less than 0.20% phosphorusand the balance mostly iron was cast into a camshaft 10. After shake outand cleaning the camshaft 10 was heated at temperatures of from about1250 F. to about 1300D F. for about 31/2 hours to produce incipientspheroidization of pearlite and lower matrix hardness below Rockwell C.Of course, this temperature range is below the austenite region for thisalloy.

Center holes 14 were drilled in the ends of the camshaft to facilitatethe final lobe hardening heat treatment.

The camshaft 10 was then subjected to a post-annealing lobe hardeningstep. rl`he lobe surfaces 12 were ame heated into the austenite regionand then rapidly cooled to produce a hard martensite structure'. Macrohardness produced 'by this surface hardening of the lobe should liebetween 4S and 58 Rockwell C. This hardness is made more easilyobtainable by the homogenizing effect of the pre-hardening anneal.

After the surface hardening operation various surfaces of the cam-shaftwere machined as discussed in detail above.

While this invention has been described in terms of a certain preferredembodiment, it is to be understood that other applications will beapparent to those skilled in the art and are within the scope of thisinvention as defined by the following claims.

We claim:

1. A method of improving the durability and machinabilitycharacteristics of a carbidic cast iron article containing 10% by weightor more primary complex iron carbides which comprises annealing the castiron casting at a suitable temperature in the temperature range of 300Fahrenheit degrees immediately below the lower critical temperature ofsaid carbidic cast iron for a time sufficient to produce incipientspheroidization of pearlite and to lower matrix hardness below 30Rockwell C and subsequently cooling the casting to about normal roomtemperature whereby said casting may subsequently be machined at aneconomical rate and a surface of said article may more readily behardened, said cast iron containing an element, which promotes theformation of hard complex iron carbides, taken from the group consistingof chromium, vanadium, molybdenum, columbium, manganese, tungsten andzirconium.

2. A method to improve the durability and machinability of a carbidiccast iron camshaft containing 10% or more by weight primary complex ironcarbides Which comprises casting the camshaft, cooling the camshaftcasting and removing the mold, cleaning said casting, annealing saidcamshaft casting at a suitable temperature in the temperature range of300 Fahrenheit degrees immediately below the lower critical temperatureof said carbidic cast iron for a time sufficient to produce incipientspheroidization of pearlite and to lower matrix hardness below 30Rockwell C, air cooling said camshaft, surface hardening the lobes ofsaid camshaft by heating said lobe surfaces to a temperature within theaustenite region and subsequently rapidly cooling to produce a hardmartensite structure and machining predetermined surfaces of Saidcamshaft other than said lobe surfaces, said camshaft alloy (having beeninoculated with) containing a carbide forming element taken from thegroup consisting of chromium, vanadium, molybdenum, manganese,columbium, tungsten and zirconium.

3. A method to improve the durability and machinability of a cast ironcamshaft which comprises casting the camshaft, cooling said camshaftcastin-g and removing the mold, cleaning said casting, annealing saidcamshaft at a temperature from about 1250 F. to about 1300o F. for atleast about 21/2 hours to produce incipient spheroidization of pearliteand to lower matrix hardness below 30 Rockwell C, air cooling saidcamshaft, surface hardening the lobes of said camshaft by heating thelobe surfaces to a temperature within the austenite region andsubsequently rapidly cooling to produce a hard martensite structure, andmachining predetermined surfaces of said camshaft other than said lobesurfaces, said camshaft alloy comprised of 3.25-3.50% carbon, 2.20-2.45%silicon, G60-0.80% manganese, 1.301.50% chromium, OAG-0.60% molybdenum,and the balance substantially all iron.

4. A method of improving the durability and machinabilitycharacteristics of a carbidic cast iron article containing 10% or moreby weight primary complex iron carbides which comprises annealing thecast iron casting at a suitable temperature in the temperature range of300 Fahrenheit degrees immediately below the lower critical temperatureof said carbidic cast iron for a time suflicient to produce incipientspheroidization of pearlite and to lower matrix hardness below 30Rockwell C and subsequently cooling the casting to about norm-al roomtemperature whereby said casting may subsequently be machined at aneconomical rate and a surface of said article may more readily behardened, said cast iron comprising by Weight 3.25-3.50% carbon,L20-2.45% silicon, a carbide forming element taken from the groupconsisting of chromium, vanadium, molybdenum, manganese, columbium,tungsten and zirconium in an amount sufiicient to produce said 10% ormore by weight primary complex iron carbides and the balancesubstantially all iron.

5. A pearlitic carbidic cast iron article containing 10% ReferencesCited UNITED STATES PATENTS 11/1950 Seng 14S- 12.4 X

OTHER REFERENCES Physical and Engineering Properties of Cast Iron,Angus, 1960, The British Cast Iron Research Asso., relied on pp. 357-367and 424-428.

CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT oFFIcE CERTIFICATE OF CORRECTION Patent No.3,384,515 May 21, 1968 Allen D. Ackerman et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 43, "lodes" should read lobes Column 4, 11ne 5l, cancel"[havng been inoculated with)".

Signed and sealed this 16th day of September 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr. Attesting Officer

